1. Field of the Invention
The present invention relates to an air-fuel ratio detecting device for detecting the air-fuel ratio (A/F) of an air-fuel mixture to be supplied to an internal combustion engine.
2. Related Art
There has been proposed a linear A/F sensor utilizing the oxygen concentration cell capability and oxygen ion pumping capability of zirconia, for detecting whether the air-fuel ratio is on a leaner or richer side of a stoichiometric ratio and also for detecting the value of the air-fuel ratio (see Japanese Laid-Open Patent Publication No. 63(1988)-36140).
One conventional linear A/F sensor will be described below with reference to FIGS. 7 through 10 of the accompanying drawings. FIG. 7 shows a linear A/F sensor including a sensor cell 20 and a pump cell 21 which are shown detached from each other, and each includes a stabilized zirconia device. The sensor cell 20 and the pump cell 21 are coupled to each other through an insulation layer 22. The sensor cell 20 and the pump cell 21 have respective diffusion holes 23, 24 defined therein for passing therethrough exhaust gases from an internal combustion engine. The insulation layer 22 has a detecting cavity 25 defined therein into which exhaust gases can be introduced through the diffusion holes 23, 24 by the sensor cell 20 and the pump cell 21. The diffusion holes 23, 24 and the detecting cavity 25 jointly serve as an element for controlling the speed at which the exhaust gases are diffused. The insulation layer 22 also has a reference chamber 25a positioned below the detecting cavity 25 in spaced-apart relation thereto, with the reference chamber 25a being defined between the sensor cell 20 and the pump cell 21. A reference gas such as atmospheric air is introduced into the reference chamber 25a through a communication hole (not shown). As shown in FIG. 8, the sensor cell 20 has porous electrodes 26, 27 of platinum, and the pump cell 21 has porous electrodes 28, 29 of platinum, with the electrodes 26, 27, 28, 29 doubling as a catalyst. The sensor cell 20 has an electric heater 30 for heating itself to a temperature range, e.g., 800.degree..+-.100.degree. C. in order to keep the sensor cell 20 active.
The sensor cell 20 functions as a conventional O.sub.2 sensor for developing an electromotive force if there is an oxygen concentration difference between the electrodes 26, 27. The pump cell 21 also has the same properties as the sensor cell 20, and serves to pump oxygen from a negative electrode to a positive electrode when an electric current (pump current Ip) is caused to flow between the electrodes 28, 29.
A control assembly 31 detects an electromotive force Vs developed by the sensor cell 20, and also controls the pump current Ip through a feedback loop in order to keep constant the electromotive force Vs, i.e., in order to keep an oxygen concentration corresponding to a stoichiometric ratio in the detecting cavity 25 or the diffusion holes 23, 24. Since the pump current Ip continuously varies with respect to the air-fuel ratio, as shown in FIG. 9, the air-fuel ratio can be calculated from the pump current Ip.
More specifically, the control assembly 31 includes a comparator 1 and an integrator amplifier 2 with positive and negative power supplies. The comparator 1 compares the electromotive force Vs and a reference voltage Vref corresponding to the stoichiometric ratio. The output signal from the comparator 1 is integrated by the integrator amplifier 2, whose integral output signal is applied as the pump current Ip to the pump cell 21 through a resistor 5. At this time, a voltage drop across the resistor 5 is detected by a current detector 3 which produces a voltage signal commensurate with the pump current Ip. Therefore, the pump current Ip is detected indirectly by the current detector 3. The output signal of the current detector 3 is applied to an adder 4 which then produces an output signal Vout, in the range of from 0 to 5 volts, representing the air-fuel ratio, according to the following equation: EQU Vout=G.Ip+Vstp
where G is the current-to-voltage conversion gain of a current-to-voltage converter which is composed of the resistor 5 and the current detector 3, and Vstp is a step-up voltage in the range of from 0 to 5 volts.
In the conventional system shown in FIG. 8, the voltage drop across the resistor 5 is applied to a current inversion detector 6 to detect the direction in which the pump current flows, for thereby producing a stoichiometric air-fuel ratio Vstc (see FIG. 10).
With the linear A/F sensor, the pump current Ip is of a value corresponding to the concentration of O.sub.2 (which, if higher, makes the air-fuel mixture leaner) in the exhaust gas, and the concentrations of H.sub.2, CO (which, if higher, make the air-fuel mixture richer), and has a characteristic as indicated by the following equation (1): EQU Ip.varies.(K.sub.1.T.sup.0.75.S/L+K.sub.2.T.sup.-0.5.Pg.S/L) (1)
where K.sub.1 and K.sub.2 are constants that vary depending on the structure of the linear A/F sensor, T the absolute temperature, Pg the partial pressure of oxygen in the measured exhaust gas, S the cross-sectional area of the diffusion hole in the gas diffusion limiting layer, and L the thickness of the gas diffusion limiting layer.
It is known that if the linear A/F sensor is of such a structure as to mainly diffuse the gas with molecules, then the constant K.sub.1 is larger than the constant K.sub.2, making the air-fuel ratio information highly dependent on the temperature, and if the linear A/F sensor is of such a structure as to mainly diffuse the gas with minute holes, then the constant K.sub.2 is larger than the constant K.sub.1, making the air-fuel ratio information highly dependent on the pressure.
In the case where the linear A/F sensor of a structure to mainly diffuse the gas with minute holes is employed, it has been found that, as shown in FIG. 9, the pump current vs. air-fuel ratio curve b which is plotted under a reference pressure acting on the sensor in a reference environment widely deviates from the pump current vs. air-fuel ratio curve a which is plotted under a measured pressure on the sensor on both sides of the stoichiometric air-fuel ratio (where the pump current Ip is zero), with the deviation being greater as the absolute value of the pump current Ip is greater.
With the linear A/F sensor which is of a structure to mainly diffuse the gas with minute holes, no consideration has heretofore been given to the dependency of the air-fuel ratio information produced by the sensor, on the pressure and temperature. It has been customary to control the air-fuel ratio of the fuel injection device through a feedback control loop based on the air-fuel ratio information which is not corrected.
Accurate control of the air-fuel ratio so that it reaches a target value while the internal combustion engine is in operation is very important for improved fuel economy, increased engine output power, stabler idling engine speed, purified exhaust emission, and improved drivability. However, the above conventional air-fuel ratio control process has proven unsatisfactory.